Up-regulation of voltage-gated sodium channels by peptides mimicking S4-S5 linkers reveals a variation of the ligand-receptor mechanism.

Prokaryotic Na _V channels are tetramers and eukaryotic Na _V channels consist of a single subunit containing four domains. Each monomer/domain contains six transmembrane segments (S1-S6), S1-S4 being the voltage-sensor domain and S5-S6 the pore domain. A crystal structure of Na _V Ms, a prokaryotic Na _V channel, suggests that the S4-S5 linker (S4-S5 _L ) interacts with the C-terminus of S6 (S6 _T ) to stabilize the gate in the open state. However, in several voltage-gated potassium channels, using specific S4-S5 _L -mimicking peptides, we previously demonstrated that S4-S5 _L /S6 _T interaction stabilizes the gate in the closed state. Here, we used the same strategy on another prokaryotic Na _V channel, Na _V Sp1, to test whether equivalent peptides stabilize the channel in the open or closed state. A Na _V Sp1-specific S4-S5 _L peptide, containing the residues supposed to interact with S6 _T according to the Na _V Ms structure, induced both an increase in Na _V Sp1 current density and a negative shift in the activation curve, consistent with S4-S5 _L stabilizing the open state. Using this approach on a human Na _V channel, hNa _V 1.4, and testing 12 hNa _V 1.4 S4-S5 _L peptides, we identified four activating S4-S5 _L peptides. These results suggest that, in eukaryotic Na _V channels, the S4-S5 _L of DI, DII and DIII domains allosterically modulate the activation gate and stabilize its open state.